A vast amount of hydrocarbon, chlorinated and halogenated solvent contaminated soil as well as pesticide and herbicide contaminated soils require cleaning up. One method of removing the contaminates from the soil is to heat the soil until the contaminates are vaporized. The contaminate vapors and the decontaminated soil are then removed from the heating chamber and the process is continually repeated until all the soil has been decontaminated.
Rotary dryers and heat exchangers have been used in the past to transfer heat from a hot gas, usually the product of combustion, to the sludge or granular material which it is desired to heat or dry. These rotary drum dryers are generally of two types. Both types employ a horizontal rotating drum through which the sludge or granulated material passes. The drum is generally slightly inclined from the in-feed end to the out-feed end to cause the material being dried to progress down the drum as it rotates. Often mounted within the drum are lift flights of various types designed to agitate the material passing through the drum and/or to cast the material into a falling veil of material where it can interact with gasses passing through the drum. One type of drum heater utilizes indirect heating wherein a furnace or manifold for hot gases surrounds the central portions of the drum, thus heating the exterior of the drum which in turn heats the material passing through the interior of the drum by conduction and radiation. The other type of drum dryer employs direct heating wherein a burner or furnace at one end of the drum introduces hot combustion gases into the interior of the drum. The hot combustion gases directly transfer heat to dry moisture from the sludge or granular material progressing through the drum. The directly fired drum heaters are divided into those in which the combustion gases flow in the same direction as the granular material passing through the drum, and those in which the combustion gases flow in an opposite direction as the granular material which is being dried.
The indirectly heated drum described above is generally inefficient in that once the hot combustion gases surround the central drum, such gases are then exhausted away from the drum. Or, the hot combustion gases are passed through the drum in contact with the material being processed, thus defeating a primary purpose of indirect heating. The directly heated drums cannot address the processing of contaminated soil because the contaminate vapors must not come in contact with the high temperature combustion gases.
What is needed is a more efficient indirectly heated drum. It is to such an indirectly heated drum which the present invention is directed.